This invention relates to a sublimating and cracking apparatus for producing a beam of molecules to be deposited on a substrate, and more specifically to such an apparatus which is particularly useful with phosphorus as the source material.
In molecular beam epitaxy (MBE), molecular beams of certain elements, such as pure phosphorus, are directed onto the surface of a substrate, where they react with each other to create a layer with the desired properties. These layers are used to construct complex semiconducting structures.
Cracking effusion devices first sublimate solid source material and then "crack" it, that is, convert the vaporous material to smaller atomic species by subjecting it to extremely high temperatures. Thus, for example, a phosphorus cracker generally has a crucible for sublimating solid phosphorus, a cracker to convert the vaporous P.sub.4 phosphorus into vaporous P.sub.2 phosphorus, and a valve to control the flow of P.sub.4 phosphorus into the cracker.
Phosphorus effusion devices are constructed using either a one or two chamber design. In a single chamber design, solid red phosphorus is sublimated at about 300.degree. C. in a furnace or crucible, which is vacuum evacuated. When the red phosphorus is sublimated, it produces both red phosphorus vapor and white phosphorus vapor, which is then introduced to the phosphorus cracker by a valve before being directed to the substrate.
Such a single chamber design suffers from at least one drawback. In particular, some of the vaporous white phosphorus condenses on the walls of the chamber. At an operating temperature of 300.degree. C., the vapor pressure of white phosphorus is significantly higher than that of red phosphorus. As a result, when the valve is closed, a large pressure build-up occurs in the chamber. When the valve is then opened to the cracker, pressure bursts occur into the MBE chamber. The excess release of phosphorus into the MBE chamber is harmful to the MBE growth system. In addition, the MBE chamber requires several hours after such a pressure burst to recover to a proper working pressure.
To reduce this problem, it has been proposed to add a second chamber to the system design. The vaporous white phosphorus is purposefully condensed in a second chamber so that it deposits on the walls of the second chamber. This second chamber is independently thermostated so that the walls are cooler to encourage the white phosphorus condensation, which significantly reduces the vapor pressure within the second chamber. A valve admits the vaporous phosphorus to the cracker where P.sub.4 phosphorus is converted to P.sub.2 phosphorus.
U.S. Pat. No. 5,431,735, issued Jul. 11, 1995 to Briones, describes a phosphorus effusion cell which uses two chambers. In all but one of the embodiments shown in the Briones patent, the condensing chamber is located downstream of the sublimation chamber and upstream of the cracker, such that the phosphorus flows continuously from the sublimation chamber, through the condensation chamber, and to the cracker. In one embodiment, shown in FIG. 9 of the Briones patent, the condensing chamber is not shown to be downstream of the sublimation chamber, but rather is disposed in a "side-by-side" relationship at opposite ends of a separator tube, with a cracking tube leading to the cracker connected to the midpoint of the separator tube. While the Briones patent suggests that such an arrangement can still provide a continuous flow of phosphorus vapor without pressure build-up problems, it is unclear whether this is, in fact, the case.
In addition, other problems remain with existing two chamber phosphorus effusion cells. Although the second chamber may be vacuum evacuated, it is surrounded by air at atmospheric pressure. This pressure difference causes atmospheric gases, such as O.sub.2, N.sub.2, and Ar, to leak or diffuse into the lower pressure chambers containing the red and white phosphorus. This leakage allows contaminants to get into the phosphorus flux when the valve is opened to deposit phosphorus.
Solid-source valved cracker sources have previously used a conventional cracking tube that consists of a high temperature resistant tube (tantalum, tungsten, PBN, graphite, molybdenum) that was heated from an exterior heater. The cracking of the molecules is accomplished by heating the exterior of the tube to a high temperature so that the interior surface of the tube is hot enough to thermally decompose or crack the molecules into smaller species. This device uses more energy than is necessary to crack the molecules, because the heating filaments supply heat to the molecules only indirectly. As a result, the heat shielding and exterior filament are larger than necessary. Also, if a baffle assembly is inserted into the tube it is blocked from direct radiation from the hot filament by the tube. Therefore, a catalytic baffle used to reduce the temperature needed to crack the molecule is not getting as hot as the tube in crackers of this design. Therefore, this design wastes energy. Also, adding unnecessary energy, or heat, causes contaminants to be released into the vacuum growth environment. A source capable of operating with less heat-released contamination is desired.